FR2982785A1 - METHOD FOR OBTAINING AN OPHTHALMIC LENS - Google Patents

Abstract

This method comprises: the step of selecting a material which exhibits a pitting threshold during a machining step of a finishing surface (20) over a depth of material removal (D) of 0.07 mm which is at least 15% higher than the pitting threshold in a machining step of a finishing surface at a material removal depth (D) of 0, 22 mm; the step of selecting a set of finishing pass depth (A) between 0.015 mm and 0.075 mm; and the step of selecting a finishing feed setpoint between 85 and 99% of the pitting threshold for the material removal depth (D) given by the sum of the finishing pass depth setpoint. selected (A) and rough roughness (B ').

Description

FIELD OF THE INVENTION The invention relates to obtaining an ophthalmic lens from a puck made of a material that can be machined. BACKGROUND ART It is known that in order to obtain from such a puck an ophthalmic lens having the desired optical characteristics, for example to correct the vision of a myopic, astigmatic and / or presbyopic carrier, a surfacing step is used. by machining a face of the puck. This surfacing step successively comprises a step of machining a rough surface and a step of machining a finishing surface. The machining step of a blank surface is carried out with a machine comprising a roughing tool, for example as described in US Pat. No. 5,938,381, forming a blank surface having a blank groove. having an arcuate profile whose radius of curvature is fixed by the shape of the roughing tool and having a pitch defined by a pre-feed advance given to the machine. The step of machining a finishing surface is carried out with a machine comprising a finishing tool, for example as described in the international application WO 2006/097607, to which corresponds the US patent application US 2009 / 0022554, this finishing tool forming a finishing surface by digging the roughing surface, to a depth defined by a finishing depth setpoint given to the machine, forming a finishing groove having a pitch defined by a set point Finishing feed given to the machine.

The machining surfacing step is followed by a polishing step which gives the optical rendering of the surface without modifying its shape.

OBJECT OF THE INVENTION The invention aims to improve the surfacing step by machining a face of a puck intended to become an ophthalmic lens. To this end, it proposes a process for obtaining an ophthalmic lens, comprising: a step of supplying a puck made of a material that can be planed by machining; and a step of surfacing by machining a face of said puck successively comprising a step of machining a rough surface and a step of machining a finishing surface, said step of machining a surface roughing machine being implemented with a machine comprising a roughing tool forming a blank surface having a blank groove having a circular arc profile whose radius of curvature is fixed by the shape of the tool d roughing and having a pitch defined by a blank advance setpoint given to said machine; said step of machining a finishing surface being carried out with a machine comprising a finishing tool forming a finishing surface by digging said roughing surface, to a depth defined by a given finishing depth of depth setpoint. to said machine, forming a finishing groove having a pitch defined by a finishing feed command given to said machine; characterized in that: - said step of providing a puck comprises the step of selecting for said puck a said material which has a threshold of pitting appearance during a said step of machining a finishing surface at a material removal depth of 0.07 mm which is at least 15% higher than the pitting threshold in a said step of machining a finishing surface to a depth of removal of material of 0.22 mm, each said pitting threshold being the finishing advance setpoint from which pits occur for a predetermined depth of material removal, each said depth of removal of material. material being the sum of the set finishing depth setpoint and the rough roughness B 'given by the relation: C' 'B' - 8R 'with: - C' which is the advance of roughing or pitch of the roughing groove; and - R 'which is the radius of curvature of the roughing tool or roughing groove; said step of machining a finishing surface comprises the step of selecting said setpoint of depth of finishing pass between 0.015 mm and 0.075 mm; and said step of machining a finishing surface comprises the step of selecting said finishing advance set point between 85 and 99% of the pitting threshold for the depth of material removal given by the sum of the set finishing depth setpoint and the rough roughness B '.

The invention is based on the discovery that for some materials, as the depth of material removal decreases, the pitting threshold increases; and in particular for some materials the increase is at least 15% when going from a material removal depth of 0.22 mm to a material removal depth of 0.07 mm.

The invention is also based on the observation that the main defects that must be removed during the machining operation of a finishing surface, are the cracks that are created during the machining operation of a roughing surface below the roughing surface and with current machining and roughing tools, a finishing depth of between 0.015 mm and 0.075 mm, which is much smaller than the depths of pass used classically, remains lower than the crack depth. By thus choosing a particularly small depth of pass and placing itself as close as possible to the pitting threshold, the step of machining a finishing surface can be carried out in a much smaller time, since it is at least 15% lower than the machining steps of a conventional finishing surface.

According to preferred features: said step of providing a puck includes the step of selecting for said puck a Polythiourethane; said polythiourethane is MR7; said step of machining a finishing surface is carried out with a finishing tool configured to form a finishing groove having a radius of curvature of between 1.9 and 2.1 mm; said set of finishing depth is between 0.03 and 0.06 mm; while said finishing advance set point is between 0.076 and 0.102 mm / revolution; said step of machining a finishing surface is carried out with a finishing tool configured to form a finishing groove having a radius of curvature of between 4.9 and 5.1 mm; said set of finishing depth is between 0.03 and 0.06 mm; while said finishing advance set point is between 0.121 and 0.160 mm / revolution; said polythiourethane is MR8; said step of machining a finishing surface is carried out with a finishing tool configured to form a finishing groove having a radius of curvature of between 1.9 and 2.1 mm; said set of finishing depth is between 0.03 and 0.06 mm; while said finishing advance set point is between 0.055 and 0.074 mm / revolution; and / or - said step of machining a finishing surface is carried out with a finishing tool configured to form a finishing groove having a radius of curvature of between 4.9 and 5.1 mm; said set of finishing depth is between 0.03 and 0.06 mm; while said finishing advance set point is between 0.088 and 0.117 mm / revolution. According to other preferred features, said step of machining a blank surface and said step of machining a finishing surface are carried out with the same machine, and said step of machining a surface for roughing comprises the step of selecting for said rough advance set point a value minimizing a total time T of implementation, by said machine, of the machining step of a blank surface and of the machining step of a finishing surface.

The implementation of these characteristics allows the surfacing step of the method according to the invention to be particularly fast. According to preferred features, said step of selecting for said rough advance setpoint a value minimizing a total time T 5 comprises: the step of modeling the total time T by the relation: T = K1 + K2 + K3 C ' C with: K1 which is a constant grouping the times spent by the machine for operations other than the step of machining a blank surface and the step of machining a finishing surface; - K2 which is a constant related to the machining step of a blank surface; K3 which is a constant related to the machining step of a finishing surface; and - C which is the rough feed or the pitch of the finishing groove; the step of calculating for each of n rough advance setpoint values, the respective total machining time T, with for each i th roughing advance setpoint, for i taken from 1 to n, the i-th finishing work set point 20 calculated by carrying out said step of selecting the finishing feed set point, by taking the depth of material removal given by the sum of the finishing pass depth setpoint. selected and the rough roughness B 'corresponding to the i-th draft advance setpoint; and 25 - the step of selecting, as the rough advance setpoint, the same roughing advance setpoint and as the finishing advance setpoint the mth finishing feedset, with m being the i for which the total time T is the smallest. According to preferred features, said step of machining a blank surface is carried out with a roughing tool configured to form a blank groove having a radius of curvature of between 33 and 34 mm; while said rough advance set point is between 1.16 and 5.18 mm / revolution. According to other preferred features, as being simple and convenient to implement, said rough groove and said finishing groove are spiral. BRIEF DESCRIPTION OF THE DRAWINGS The description of the invention will now be continued by the detailed description of exemplary embodiments, given below by way of illustration and not limitation, with reference to the accompanying drawings. In these drawings: - Figure 1 is a front view of a finishing tool and a tool holder that includes a machining machine of a face of a pallet of organic material intended to become after machining a ophthalmic lens; - Figure 2 is a view showing the finishing tool according to the section illustrated by I1-11 in Figure 1, during machining of a face of the puck; - Figure 3 is a very schematic view showing the groove made by the finishing tool on this face of the puck; FIG. 4 is a view taken as indicated by IV-IV in FIG. 3, the bottom line schematically illustrating the surface after machining, the top line illustrating the initial surface of this face of the puck, assuming that it is uniform. ; FIG. 5 is a very schematic view of a machining machine configured to operate on a face of a puck a surfacing step successively comprising a machining step of a roughing surface and a machining step; a finishing surface; Figure 6 is a view similar to Figure 4, but with the top line showing a blank surface formed by a roughing tool; FIG. 7 is a graph on which the depth of removal of material is plotted on the abscissa during a surfacing with the finishing tool, whereas the finishing advance instruction is plotted on the ordinate, this graph representing the pitting threshold curve for material MR7; FIG. 8 is a graph similar to the graph of FIG. 7, but for material CR39, with different scales on the abscissa and on the ordinate; FIG. 9 is a graph similar to the lower part of FIG. 7, but with the ordinates which are dilated and with the addition of the practical threshold curve before occurrence of punctures; FIG. 10 is a graph on which the roughing advance setpoint is plotted on the abscissa and the total running time of the machining step of a roughing surface and step is plotted on the ordinate for machining a finishing surface, the upper curve being established for machining steps of an invariable finishing surface, the other curves being established for machining steps of a finishing surface in accordance with the invention, for different values of finishing pass depth; and FIG. 11 is a graph similar to FIG. 9, but for material MR8. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS The finishing tool 10 illustrated in FIG. 1 and the machining machine of which it forms part are here as described in the international application WO 20 2006/097607, to which corresponds the patent application. American US 2009/0022554. The finishing tool 10 is generally circular in shape. It has a leading face 11, a flank face 12 and a rear face 13. The junction between the leading face 11 and the flank face 12 forms a cutting edge 14. The tool 10 is fixed on a tool holder 15 so that the cutting edge 14 is available on at least a portion of the circumference of the tool 10 for machining a puck 16 (Figures 2 and 3) intended to become after machining of its face 17, an ophthalmic lens. The machine of which the finishing tool 10 is part is configured to rotatably drive the puck 16 and to slidably drive the tool 10 in a direction parallel to the axis about which the puck 16 is rotated and direction transverse to this axis of rotation.

The sliding drive members of the tool in these two directions are synchronized with the drive member to rotate the puck to be able to machine a complex surface, including one of the faces of a spectacle eyeglass lens myopic, astigmatic and presbyopic.

FIG. 2 shows the tool 10 machining the face 17 of the pallet 16: the tool 10 penetrates the material of the pallet 16 to a depth defined by a set of depth of pass given to the machine, the face 16 being machined with the leading face 11 of the tool 10 which produces chips by its advance in the material.

More specifically, the tool 10 digs a groove 18 (FIGS. 3 and 4) in a spiral having a step defined by an advance instruction given to the machine. In Figure 3, the groove 18 is shown very schematically by its bottom lines. Note that to simplify the drawings, the groove 18 has been shown in FIG. 3 with a much smaller number of turns than the groove formed in practice by the finishing tool 10. FIGS. 2 and 4 show the initial surface 19 of the face 17 of the pallet 16 and the surface 20 after machining by the finishing tool 10. Note that in practice the surfacing step of the face 17 comprises successively a machining step of a rough surface with a roughing tool and a step of machining a finishing surface with the finishing tool, but here, to simplify the explanations, it is assumed in a first step that the machining of the face 17 of the Pallet 16 was made with the finishing tool 10 on a uniform initial surface 19.

Because of the generally circular shape of the finishing tool 10, the groove 18 has an arcuate profile whose radius of curvature R is fixed by the shape of the tool 10. Here, the tool 10 being of generally circular shape, the radius of curvature R of the groove 18 is the radius of the tool 10. In FIG. 4, in addition lines illustrating the surfaces 19 and 20, there is shown the depth of pass A, the roughness B and step C of groove 18.

As indicated above, the pitch C is defined by an advance setpoint given to the machine (the feedrate is C / revolution) for the tool 10, that is to say the setpoint d finishing progress. The roughness B is here the distance between the bottoms and the vertices of the groove 18. It is demonstrated that there exists between the pitch C, the radius of curvature R and the roughness B the following relation: C = 2-1R2 - (R - B) 2 If we neglect the term B2, we have the relation: B = c2 8R As indicated above, the tool 10 enters the material of the puck 16 to a depth defined by a set of depth of pass given to the machining machine. Due to the circular shape of the tool 10, the depth dug under the initial surface 19 is variable.

For reasons explained below, it is necessary to distinguish: - the pass depth A which is the difference between the starting reference surface of which the machine is aware and the target surface which it is intended to machine ; and the depth of material removal D which is the maximum difference between the actual starting surface and the actual surface after machining. Here, since the initial surface 19 is assumed to be uniform, the starting reference area and the actual starting area are merged; and the target surface that is intended to be machined is the surface that passes through the bottom lines of the groove 18; therefore the depth of pass A is equal to the depth of material removal D. The machine 30 shown very schematically in Figure 5 is configured to operate in full the milling step by machining the face 17 of the pallet 16, successively performing a machining step of a blank surface with a roughing tool 31 and a step of machining a finishing surface with the finishing tool 10.

The machine 30 includes the same drive member for rotating the pallet 16 to perform the machining step of a rough surface and the machining step of a finishing surface, the pallet 16 remaining in place on this member between a loading step performed before the machining step of a blank surface and an unloading step performed after the step of machining a finishing surface. The machine 30 comprises, for the roughing tool 31, drive members that are distinct from the drive members of the finishing tool 10. The kinematics used during the step of machining a surface of roughing is similar to the kinematics used during the machining step of a finishing surface. Here, moreover, the roughing tool 31 turns on itself. The roughing tool 31 is for example a rotary cutter as described in US Patent 5,938,381. Thus, the roughing tool forms a roughing surface 32 (FIG. 6) having a spiral blank groove 33 having an arcuate profile whose radius of curvature is fixed by the shape of the tool. 1 and 6, in addition to lines illustrating the blank surface 32 and the finishing surface 20, the depth is shown in FIG. Finishing pass A, the material removal depth D, the roughness B ', the pitch C' and the radius of curvature R 'of the rough groove 33. The radius of curvature R' and the pitch C '( and therefore the rough advance set point) are much larger than the radius of curvature R and the pitch C of the finishing groove 18.

The roughness B 'of the rough groove 33 is given by the relation: 8R' The finishing pass depth A, which is equal to the finishing depth setpoint given to the machine 30, is the distance between the the target surface of the machining step of a roughing surface (which becomes the starting reference area of which the machine is aware for the machining step of a finishing surface) and the target surface of the machining step of a finishing surface. This is in practice the difference between the surface that passes through the bottom lines of the rough groove 33 and the surface that passes through the bottom lines of the finishing groove 18. The depth of material removal D is the maximum distance that can exist between the rough groove 33 and the finishing groove 18. In practice, this is the distance between the bottoms of the groove 18 and the ridges of the groove 33. As can be seen on In Fig. 6, the depth of material removal D is the sum of pass depth A and rough roughness B '. It is known that the main defects likely to appear on the surface machined by the finishing tool 10, that is to say the finishing surface, are chips or tearing material, generally called pits. Pitting occurs when the depth of material removal becomes too great. It should be noted that the work resulting from the present invention has established that it is not sufficient to take into account the only depth of pass A but that it is also necessary to use the rough roughness B 'for 20 take into account the actual physical phenomenon, which is related to the material removal depth D. Curve 35 of FIG. 7 represents the pitting threshold for material MR7. On the abscissa is the depth of material removal D, expressed in mm. The ordinate is carried the set of finishing advance, expressed in mm / revolution. This curve was established under the predefined experimental conditions: here, the target roughing surface (which passes through the bottom lines of groove 33) is a plane and the target finishing surface (which passes through the bottom lines of the groove 18) is a plane parallel to the target blank surface; the roughness B 'of the roughing surface 32 is set at 0.02 mm, the radius of curvature R of the finishing tool 10 and thus of the groove of the finishing surface 20 is 2 mm with an angle of cutting (inclination of the leading face 11 relative to the normal of the surface to be machined) which is zero, as shown in Figure 2. The different tests were performed on the same machine with the same roughing tools 31 and finishing 10. It is found, and it is a discovery on which the present invention is based, that when the depth of removal of material decreases, the threshold of appearance of pitting increases very substantially. To allow comparisons, the curve 36 shown in FIG. 8 was established in the same way as the curve 35, but for the material CR39. It can be seen that the pitting threshold does not vary with the depth of material removal, the threshold remaining at 0.04 mm / revolution between a material removal depth of 0.5 mm and a depth of 0.5 mm. removal of material very close to 0. In practice, in the ophthalmic lens manufacturing laboratories, a finishing feed setpoint of 0.033 mm / revolution is used for material CR39 regardless of the depth of finishing pass setpoint, which is usually 0.2 mm. As can be seen better in FIG. 9, which takes up the part of the curve 35 which is seen below in FIG. 7 with the ordinates which are dilated, for a material removal depth of 0.22. mm, the threshold of occurrence of pits is about 0.069 mm / revolution while for a material removal depth of 0.07 mm, the pitting threshold is about 0.091 mm / revolution. The threshold for 0.07 mm is thus 32% higher than the threshold for 0.22 mm.

Curve 37 of FIG. 9 gives the practical threshold before occurrence of pitting, that is to say the curve giving the maximum values of the finishing advance setpoint that can be used in practice. In general, for finishing feed values from curve 35, it is certain that there are pits; for the finishing feed values up to the curve 37, it is certain that there are no pits; and for the finishing feed values between curve 37 and curve 35, pitting may or may not occur.

The offset between the pitting threshold curve and the practical threshold curve before pitting occurs depends on the precision of the machine used and the wear margin that is expected for the tool. finishing (this margin of wear is related to the frequency of renewal of the tool). In the illustrated example, the machine is of good precision and the finishing tool 10, diamond, is changed after machining the finishing surface of a thousand ophthalmic lenses. The offset between the curves 35 and 37 is not constant because it also depends on the amount of pitting that appears: the more stitches the more the offset is important. For a material removal depth of 0.22 mm, the practical threshold before pitting is 0.063 mm / revolution while for a material removal depth of 0.07 mm, the practical threshold before pitting is 0.087 mm / revolution. The threshold for 0.07 mm is thus 38% higher than the threshold for 0.22 mm. The following values were also found by the tests on which the present invention is based: Depth Threshold of appearance of practical Threshold before removal of pits Pitting appearance Material (mm) (mm / revolution) (mm / revolution) 0.035 0.111 0.101 0.060 0.093 0.091 0.080 0.090 0.082 0.095 0.086 0.078 It should be noted that it has been heretofore considered that the different organic materials in which ophthalmic lenses are made have a behavior similar to material CR39 with regard to the machining of ophthalmic lenses. a finishing surface; that consequently it has hitherto been considered that for the material MR7 the pitting threshold is independent of the depth of pass set point; and that the conventional finishing feedrate of 0.063 mm / turn should be used regardless of the finishing pass depth, which is generally set, as well as for material CR39, at 0.20 mm. In fact, as shown by the discovery on the basis of the present invention, for the material MR7, the finishing advance can be very substantially increased, and thus the machining step of a finishing surface, reducing the depth of pass. The constraints that are then to be respected are the following: (i) the depth of pass must be sufficiently large to eliminate the defects related to the machining of a rough surface; and (ii) the finishing feed must not exceed the practical threshold before pitting. The constraint (ii) is satisfied when the values given by the curve 37 are chosen.

With regard to the constraint (i), it should be known that during the machining operation of a rough surface, cracks 34 are created under the rough surface 32 which propagate up to at a depth E below the surface 32, and more precisely under the bottoms of the groove 33. The cracks 34 are the main defects that must be removed during the machining operation of a finishing surface. It must also be possible to make up for the slight differences in shape that may appear between the roughing target surface and the surface passing through the bottom lines of the groove 33. Traditionally, for the material CR39, the pass depth A is chosen equal 0.20 mm, which is considerably larger than the crack depth E. Here, for the MR7 material, the pass depth A is chosen close to the crack depth E. In this case the pass depth A, defined by the set of finishing pass depth given to the machine 30, is chosen between 0.015 mm and 0.075 mm. In general, for a machine of good precision with the roughing tool 31 which is a rotary milling cutter whose radius of curvature R 'is 30 mm, with the roughing advance which is 2.19 mm / revolution (the roughness D 'is then 0.02 mm), a pass depth of 0.05 mm is a suitable value. At this depth of 0.05 mm a material removal depth D of 0.07 mm is achieved under the selected experimental conditions, where the roughness B 'is 0.02 mm. The maximum value for the finishing feed, given by curve 37 of FIG. 9, is then 0.087 mm / revolution. If we take the example, for a puck 16 having a diameter of 70 mm, machining on the face 17 of the pallet 16 of a toric surface whose radii of curvature are 80 mm and 300 mm the machining time (machining step of a roughing surface and machining step of a finishing surface) is 42.6 s when using for the machining step of a finishing surface the conventional parameters of 0.20 mm for pass depth and 0.063 15 mm / turn for finishing feed; while with the above parameters (pass depth 0.05 mm and finishing feed 0.087 mm / turn), the total machining time is 35.2 s, a decrease of 17% (in both in this case, the same parameters have been retained for the machining step of a blank surface, in particular a roughing advance of 2.19 mm / revolution). To further minimize the processing time of the machining operation of a finishing surface, it is also possible to reduce the rough roughness B 'by acting on the rough feed value, c That is, in step C 'of the rough groove 33. As shown by the formula recalled above, the rough roughness B' varies as the square of the rough feed or the pitch. VS'. The reduction in the implementation time of the machining step of a finishing surface provided by the reduction of the rough roughness B 'results in an increase in the time of completion of the machining step. a roughing surface since the roughing advance is decreased. We will now describe a method for finding out what is the rough advance for which the total machining time (machining step of a roughing surface and machining step of a finishing surface) is minimized. The total time is modeled by the following relation: T = K1 + K2 + K3 C 'C With: - K1 which is a constant grouping the times spent by the machine 30 for operations other than the machining step of a machine. roughing surface and the machining step of a finishing surface; - K2 which is a constant related to the machining step of a blank surface; and - K3 which is a constant related to the machining step of a finishing surface. It should be noted that the constant K3 depends on the geometry of the surface to be machined: it will be larger the more the surface is complex.

It should also be noted that the constants K2 and K3 depend on the diameter of the pallet 16. In order to know the constants K1, K2 and K3, the machine 30 is made to record empty, giving it the same diameter for each survey for the same time. puck 16 and the same surface geometry to achieve.

Vacuum time readings are carried out, for example, with the following tests: - standard roughing advance and standard finishing feed; - roughing advance multiplied by 2 and standard finishing feed; and - standard roughing advance and finishing feedrate multiplied by 2. By solving a simple linear system of three equations (one per test) with three unknowns (the constants K1, K2 and K3), the constants K1, K2 and K3 can be determined. The total machining time T is then calculated for a certain number of rough feed values, taking the same pass depth A chosen as explained above, that is to say, as close as possible. As far as possible from the crack depth E. On the graph of FIG. 10, the abscissa is plotted on the abscissa and the total time T. on the ordinate.

The curves 40, 41 and 42 were respectively established for a finishing pass depth A of 0.02 mm, 0.05 mm and 0.10 mm. The blank advance values taken are, for example, as shown in Fig. 10: 0.5 mm / revolution, 1.5 mm / revolution, 2.2 mm / revolution, 3.5 mm / revolution, 4 , 9 mm / revolution and 6.9 mm / revolution.

For each blank advance value taken, from the radius of curvature R 'and from the relation given above, the rough roughness B' was determined. The sum of the rough roughness B 'and the depth of pass A gives the depth of material removal D.

From the depth of material removal D, it was determined with the curve 37 of Figure 9, the finishing feed. The finishing feed being thus determined, the total machining time T is calculated with the above relation. It can be seen in FIG. 10 that for the curve 40 (finishing pass depth A of 0.02 mm), the total machining time T the smallest is for the rough feed value is 1.5 mm / revolution; and that for the curves 41 and 42 (finishing pass depth A respectively of 0.05 mm and 0.10 mm), the total time T the small is for the rough advance value of 2.2 mm /tower.

A first approach is to select the rough feed value for which the total time T is the smallest (1.55 mm / revolution for curve 40 and 2.2 mm / revolution for curves 41 and 42) and to take the corresponding finishing advance value. Another approach is to continue the calculations, taking new values close to the value for which the total time T is the smallest, in order to refine the determination of the optimal rough advance value.

Here, for curve 41 (finishing pass depth A 0.05 mm), there is a minimum for the total machining time T at the rough feed rate of 1.90 mm / revolution. The corresponding value of the finishing feed is 0.090 mm / revolution. If we take again the example, for a puck 16 of 70 mm of diameter, surfacing of the face 17 to obtain a toric surface whose radii of curvature are of 80 mm and 300 mm, for which it was seen above that the total machining time with the conventional parameters is 42.6 s (rough feed rate of 2.19 mm / revolution, finishing depth A of 0.20 mm and feed advance). finishing of 0.063 mm / revolution) while the total machining time with action on the single machining step of a finishing surface is 35.2 s (roughing feed of 2.19 mm / revolution, depth Finishing pass A of 0.05 mm and finishing feed of 0.087 mm / revolution), the optimum values are achieved (roughing feed of 1.90 mm / revolution and finishing feed of 0.090 mm / revolution ) a total machining time T of 34.8 s. To allow comparison, curve 43 of FIG. 10 gives the total machining time while keeping the finishing pass depth and the finishing advance mentioned above (respectively 0.05 mm and 0.087 mm / revolution). It can be seen that in this case, the total time decreases regularly with the roughing advance, without going through a minimum, and that it is always greater than the total time T corresponding to the same finishing pass depth (curve 41). . It should be noted that other tests were carried out with a roughing tool 31 configured to form a rough groove having a slightly larger radius of curvature, namely 33.5 mm instead of 30 mm; and then the total time T is found to be the smallest for a rough feed rate of 1.94 mm / revolution with the total machining time T which is then 34.1 s.

In general, for a roughing tool configured to form a rough groove having a radius of curvature R 'of between 33 and 34 mm, it is advantageous to use blank advance instructions between 1.16 and 5.18 mm / turn. Figure 11 is a graph similar to Figure 9, but for MR8 material.

Curve 45 represents the threshold of appearance of pits while curve 47 represents the practical threshold before occurrence of punctures. For a material removal depth of 0.22 mm, the pitting threshold is 0.052 mm / revolution and the practical threshold before pitting is 0.049 mm / revolution while for With a material of 0.07 mm, the pitting threshold is 0.069 mm / revolution and the practical threshold before pitting is 0.059 mm / revolution. The threshold of appearance of pits for 0.07 mm is thus 33% higher than the threshold for 0.22 mm.

The practical threshold before occurrence of pits for 0.07 mm is 20% higher than the practical threshold before pitting occurs for 0.22 mm. The following values were also found by the tests underlying the present invention: Depth Threshold of appearance of practical Threshold before removal of pits Stitching appearance Material (mm) (mm / revolution) (mm / revolution) 0.035 0.083 0.066 0.060 0.072 0.060 0.080 0.065 0.057 0.095 0.061 0.055 In the examples described, the material of which puck 16 is made is MR7 or MR8, for which it has been found that the pitting threshold increases substantially when decreases the depth of material removal. Alternatively, the material is different from MR7 or MR8, for example another Polythiourethane or other organic material or not.

The experimental conditions in which the pitting thresholds have been established include machining a target blank surface which is a plane and a machining step of a target finish surface which is a plane. parallel to the target blank surface. In a variant, the target surfaces are different from planes, for example parallel spheres. In the examples described above, the finishing tool 10 has a radius of curvature of 2 mm. Alternatively, this radius of curvature is different, for example 5, 7 or 12 mm. It will be noted that with a larger radius of curvature, the pitting appearance threshold curve is shifted upwards, that is to say that for the same depth of pass, the threshold of occurrence of punctures is upper. It is therefore possible, by increasing the radius of curvature of the finishing tool, to increase the finishing feedrate and thus to reduce the time required for the machining step of a finishing surface. For example, for the material MR7, if we take a finishing tool with a radius of curvature of 5 mm, we have the following values: Depth Threshold of appearance of threshold Practical before removal of bites appearance of punctures Material (mm) (mm / revolution) (mm / revolution) 0.035 0.177 0.176 0.060 0.152 0.148 0.080 0.142 0.134 0.095 0.137 0.131 For material MR7, when using a finishing tool with a radius of curvature of 7 mm, the following values are available: Depth Threshold of appearance of Threshold practice before removal of punctures appearance of stitching material (mm) (mm / turn) (mm / rev) 0.035 0.210 0.208 0.060 0.180 0.175 0.080 0.168 0.158 0.095 0.162 0.155 For the material MR8, if we take a finishing tool with a radius of curvature of 5 mm, we have the following values: Depth Threshold of appearance of threshold Practical before removal of punctures appearance of pitting material (mm) (mm / revolution) (mm / revolution) 0.035 0.126 0.103 0.060 0.113 0.095 0.080 0.103 0.089 0.0 95 0.097 0.087 For material MR8, if a finishing tool with a radius of curvature of 7 mm is used, the following values are used: Depth Threshold of appearance of threshold Practical level before removal of punctures appearance of pitting material (mm) (mm / rev) (mm / rev) 0.035 0.149 0.122 0.060 0.133 0.113 0.080 0.122 0.105 0.095 0.115 0.103 In the examples described above, the cutting angle is zero. In a variant, the angle of cut is different, in particular negative. Note that with a negative cutting angle, puncture threshold values are higher.

In the examples described above, the cutting edge 14 (Fig. 2) of the finishing tool is rough. Alternatively, there is an inclined facet, as described in International Application WO 2006/097607. With such an inclined facet, the threshold of occurrence of pitting is higher. In general, the tests on the basis of the present invention have shown that the values of the practical threshold before occurrence of pits are between 85 and 99% of the values of the threshold of appearance of pits. In practice, the values of interest for the depth-of-pass setpoint A are between 0.015 mm and 0.075 mm (depth of material removal D between 0.035 mm and 0.095 mm if the rough roughness B 'is 0 mm. , 02 mm); and more particularly the instructions for finishing depth of between 0.03 and 0.06 mm. With this final range of depth of finishing pass A, the following finishing feed ranges are as follows: 0.076 to 0.102 mm / revolution for material MR7 and a radius of curvature R of between 1.9 and 2.1 mm; 0.121 to 0.160 mm / revolution for the material MR7 with a radius of curvature R between 4.9 and 5.1 mm; 0.055 to 0.074 mm / revolution for the material MR8 and a radius of curvature R of 1.9 to 2.1 mm; and 0.088 to 0.117 mm / revolution for the material MR8 with a radius of curvature R between 4.9 and 5.1 mm. In general, the tests on the basis of the present invention have shown that the materials of interest for reducing the duration of the surfacing step are the materials for which the threshold of appearance of pits during the step of machining of a finishing surface to a material removal depth of 0.07 mm is at least 15% higher than the threshold of pitting occurrence during a machining step of a surface of finishing on a material removal depth of 0.22 mm. In principle, this ratio of at least 15% is likely to reach 200%. In the examples described above, the rough groove and the finishing groove are spiral. In a variant, the grooves are in parallel straight lines spaced apart from one another by a step defined by the feed advance or finishing feedrate given to the machine.

Many other variants are possible depending on the circumstances, and it is recalled in this regard that the invention is not limited to the examples described and shown.

Claims (12)

REVENDICATIONS1. Method for obtaining an ophthalmic lens, comprising: - a step of providing a puck (16) made of a material that can be machined; and a surfacing step by machining a face (17) of said puck (16) successively comprising a step of machining a blank surface (32) and a step of machining a finishing surface ( 20), said step of machining a blank surface (32) being carried out with a machine (30) having a roughing tool (31) forming a blank surface (32) having a groove blank (33) having an arcuate profile whose radius of curvature (R ') is fixed by the shape of the roughing tool (31) and having a pitch (C') defined by a set point of draft advance given to said machine; said step of machining a finishing surface (20) being carried out with a machine (30) having a finishing tool (10) forming a finishing surface (20) by digging said roughing surface (32) at a depth defined by a finishing pass depth instruction (A) given to said machine, forming a finishing groove (18) having a pitch (C) defined by a finishing feed command given to said machine ; characterized in that: - said step of providing a puck (16) comprises the step of selecting for said puck (16) a said material which has a threshold pitting appearance during a said machining step a finishing surface at a material removal depth (D) of 0.07 mm which is at least 15% higher than the pitting threshold in a said machining step of a finishing surface at a material removal depth (D) of 0.22 mm, each said pitting threshold being the finishing advance setpoint from which pits for a depth of predetermined material removal (D), each said material removal depth (D) being the sum of the finishing depth setpoint (A) used and the rough roughness B 'given by the relation: B '- C' '8R' with: - C 'which is the roughing advance or the pitch of the roughing groove; and - R 'which is the radius of curvature of the roughing tool or roughing groove; said step of machining a finishing surface (20) comprises the step of selecting said set of finishing pass depth (A) between 0.015 mm and 0.075 mm; and said step of machining a finishing surface (20) comprises the step of selecting said finishing feed set point between 85 and 99% of the pitting threshold for the depth of material removal ( D) given by the sum of the selected finishing depth setpoint (A) and the rough roughness B '. 2. Method according to claim 1, characterized in that said step of providing a puck (16) comprises the step of selecting for said puck (16) a Polythiourethane. 3. Method according to claim 2, characterized in that said Polythiourethane is MR7. 4. Method according to claim 3, characterized in that said step of machining a finishing surface is carried out with a finishing tool (10) configured to form a finishing groove (18) having a radius of curvature (R) from 1.9 to 2.1 mm; said set of finishing pass depth (A) is between 0.03 and 0.06 mm; while said finishing advance set point is between 0.076 and 0.102 mm / revolution. Method according to claim 3, characterized in that said step of machining a finishing surface is carried out with a finishing tool (10) configured to form a finishing groove (18) having a radius of curvature (R) from 4.9 to 5.1 mm; said set of finishing depth exceeds (A) is between 0.03 and 0.06 mm; while said finishing advance set point is between 0.121 and 0.160 mm / revolution. 6. Method according to claim 2, characterized in that said Polythiourethane is MR8. Method according to claim 6, characterized in that said step of machining a finishing surface is carried out with a finishing tool (10) configured to form a finishing groove (18) having a radius of curvature (R) from 1.9 to 2.1 mm; said set of finishing pass depth (A) is between 0.03 and 0.06 mm; while said finishing advance setpoint is between 0.055 and 0.074 mm / revolution. Method according to claim 6, characterized in that said step of machining a finishing surface is carried out with a finishing tool (10) configured to form a finishing groove (18) having a radius of curvature (R) from 4.9 to 5.1 mm; said set of finishing pass depth (A) is between 0.03 and 0.06 mm; while said finishing advance set point is between 0.088 and 0.117 mm / revolution. 9. A method according to any one of claims 1 to 8, characterized in that said step of machining a blank surface (32) and said step of machining a finishing surface (20) are set implemented with the same machine (30), and said step of machining a blank surface (32) includes the step of selecting for said rough advance setpoint a value minimizing a total time T of setting by said machine (30), of the step of machining a blank surface (32) and the step of machining a finishing surface (20). 10. Method according to claim 9, characterized in that said step of selecting for said rough advance setpoint a value minimizing a total time T comprises: the step of modeling the total time T by the relation: T = Kl + K2 + K3 C with: - K1 which is a constant grouping the time spent by the machine (30) for operations other than the machining step of a blank surface (32) and the step of machining a finishing surface (20); - K2 which is a constant related to the machining step of a blank surface (32); - K3 which is a constant related to the machining step of a finishing surface; and - C which is the rough feed or the pitch of the finishing groove; the step of calculating, for each of n rough draft setpoint values, the respective total machining time T, with for each i th rough advance setpoint, for i taken from 1 to n; , the i-th set of finishing feedrate calculated by implementing said step of selecting the finishing feed setpoint, by taking the depth of material removal (D) given by the sum of the depth setpoint finishing pass (A) selected and rough roughness B 'corresponding to the i-th rough feed set point; and the step of selecting as the rough advance setpoint the m-th roughing advance setpoint and as the finishing feed setpoint the mth finishing feed setpoint, with m which is the i for which the total time T is the smallest. 11. Method according to any one of claims 1 to 10, characterized in that said step of machining a rough surface is carried out with a roughing tool (31) configured to form a groove. blank (33) having a radius of curvature (R ') between 33 and 34 mm while said rough advance set point is between 1.16 and 5.18 mm / revolution. 12. Method according to any one of claims 1 to 11, characterized in that said rough groove (33) and said finishing groove (18) are spiral.